The invention generally concerns synthetic resin-coated metal pigments, a process for the production thereof and uses thereof.
Aqueous or water-based painting and printing ink systems are steadily gaining in importance due to the increasing emphasis which is being put on environmental protection. Such systems cannot use metal pigments with a reactive surface, or they can use such pigments only to a limited extent because the surface alters chemically with the passage of time, and that generally results in an adverse effect on the visual properties of the metal pigment particles, such as shine and brilliance. That applies in particular in regard to aluminum pigments which react in contact with water, with the formation of hydrogen, in particular in acid or alkaline media. However aqueous coating compositions which are of a basic nature frequently involve an alkaline environment for the metal pigment particles therein.
A number of proposals have been put forward for the purposes of stabilising the surface of metal pigments, in particular aluminum-based metal pigments, to resist the attack of chemicals and water, in particular in order to permit such pigments to be used in aqueous paint, lacquer and ink compositions which contain a binding agent. A basic proposal along those lines is concerned with inhibiting the reaction with water by the adsorption of corrosion inhibitors on the pigment surfaces. Suitable inhibitors for that purpose are special borates, phosphates, vanadates, organic phosphoric acid compounds (esters) and certain fatty acids in monomer or polymer (oligomer) form, for example dimeric acid.
However it is not possible in that way to achieve passivation which is sufficient in particular to permit the metal pigments to be used in aqueous compositions, quite apart from the fact that the visual properties of the pigments are severely impaired by the treatment with such corrosion inhibitors.
An effective passivating treatment of aluminum pigments is described in German laid-open application (DE-OS) No 36 36 183. In that procedure, chromic acid acts on the pigment surface in such a fashion as to provide for a high level of gassing stability, even in the presence of water, while retaining the good visual properties of the metal powder. As a result aluminum pigments when treated with chromic acid in that way are suitable for the production of high-grade coating compositions such as paints or lacquers, printing inks and other coating agents, which are stable in terms of storage. However the use of toxic chromic acid gives rise to various ecological problems. Furthermore, the content of chromium compounds in the pigments is undesirable in many cases.
Another line of thrust of proposals for reducing the level of reactivity of aluminum pigments is directed to coating the pigments with protective synthetic resin coatings. A requirement for effective stabilisation of the pigments in relation to chemical and thus also physical changes is a closed and strongly adhering coating on the pigment surface. Adhesion of that nature can only be achieved when the organic material is chemically bonded, also referred to as chemisorption, to the pigment particles by means of pigment-affinitive bonding or adsorption groups, for example carboxylate or phosphate groups.
For that purpose, EP-Al 0 170 474 and 0 319 971 propose a pigment coating with phosphoric acid-modified resins. Another procedure along those lines is also the two-stage process to be found in German laid-open application (DE-OS) No 36 30 356 in which the pigments are first coated with an ethylenically unsaturated carboxylic acid acting as a bonding agent and/or an ethylenically unsaturated phosphoric acid mono- or diester, whereupon a synthetic resin coating with a three-dimensional structure is formed by the addition of a monomer with at least three ethylenic double bonds, by radical polymerisation. That coating procedure in which the resins adhere to the pigment surface by way of carboxylate and/or phosphate groups admittedly results in aluminum pigments which have sufficient gassing stability in water for many purposes, but in regard to aqueous paint or lacquer and printing ink systems which are often basic the level of gassing stability is unsatisfactory because the carboxylate or phosphate adsorption groups in such systems are not sufficiently stable in regard to hydrolysis so that the synthetic resin coating at least partially detaches from the pigment surface after a relatively short storage time.
U.S. Pat. No. 4,213,886 describes the production of a synthetic resin-coated alum/hum pigment in which an acrylate resin which forms the outer coating, being based on an alkyl acrylate or alkyl methacrylate, is bonded to the pigment surface by way of silane groups. In that procedure, firstly an ethylenically unsaturated silane is adsorbed on the pigment, whereupon the double bonds in the silane are radically polymerised with an acrylic monomer to provide an acrylate coating. However even pigments when coated by means of that process cannot be used to produce aqueous paint or varnish or printing ink systems which have sufficient storage stability. Apart from the fact that, with those pigments, the adhesion of the synthetic resin coating on the pigment, which is afforded by the silane treatment, is unsatisfactory, the structure of the acrylate coating also appears to be insufficiently resistant to the penetration of water and chemicals. Admittedly, the above-indicated U.S. patent specification discloses that a hydrolysable group is to be connected to the silicon atom of the silane and that there is a relationship between hydrolysis of the silanes and the pH-value of the silane solution, but the specification does not give specific instructions for specific performance of the hydrolysis operation.